Many employers won't entertain employing a 15-year-old due to Health and Safety laws in the workplace, and the fact there are safeguarding issues as you are still classed as a child. Also, they know a 15-year-old will have GCSE's in June to work towards.
I've just seen an advertisement from a local employer that was very direct : Do not apply if you are UNDER 16 as we do not accept child labour.
Smaller places like independent shops, seasonal tourist-type work employers may take you on at weekends only, but in general, due to Health and Safety at work laws, you cannot get a job until you are 16.
All the best for your future.